Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 29.0	$ 33.8
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(9.1)	(4.1)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	19.9	29.7
Foreign Currency Transaction Gain (Loss), Unrealized	13.1	4.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	(2.9)	(1.1)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	10.2	3.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	42.1	38.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(12.0)	(5.2)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	30.1	33.5
Fixed maturity investments.
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	19.4	(30.6)
Foreign Currency Transaction Gain (Loss), Unrealized	12.2	4.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	31.6	(26.0)
Short-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	4.0	61.9
Foreign Currency Transaction Gain (Loss), Unrealized	0.5	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	4.5	61.9
Convertible fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.7	(1.1)
Foreign Currency Transaction Gain (Loss), Unrealized	0	(0.2)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.7	(1.3)
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	4.9	3.6
Foreign Currency Transaction Gain (Loss), Unrealized	0.4	0.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	5.3	4.1
Forward Contracts
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0